
the so-called interface matrices [1]}. The following recursive relations between the interface matrices, which follow immediately from the definition, will be very useful: for \(k = 1,\cdots ,d\) ,
\({\cal X}^{\le k} = \left(\mathbf {I}_{n_k}\otimes {\cal X}^{\le k-1}\right)L\left({\cal X}^k\right),\\{\cal X}^{\ge k} = \left({\cal X}^{\ge k+1}\otimes \mathbf {I}_{n_k} \right)R\left({\cal X}^k\right)^\mathsf {T},\) 
